1. Field of the Invention
The present invention relates to a high-quality gallium nitride layer-structure provided with a flat surface and a very low dislocation density selectively grown on a semiconductor substrate and a method of selectively growing the layer on the substrate.
2. Description of the Prior Art
Only few data are available on the selective growth of GaN. Therefore, there have been required information on, how the crystalline quality of GaN can be improved drastically using selective growth and a suitable layer structure.
The first study to our best knowledge was given by Y. Kato et al. (J. Crystal Growth 144 (1994) 133). They used GaN layers for patterning with a SiO.sub.2 mask as shown in FIG. 12A. The substrate 18 was sapphire (1120), 18, followed by a buffer layer of AIN 19, and a 2 .mu.m thick GaN crystal layer 20. Then a 100 nm thick SiO.sub.2 layer 21 was sputtered and stripes opened by a lithography and etching process. FIGS. 12B to 12F show processes of the successive growth of GaN on the mask in a cross sectional view. As can be seen, first a rough surface occurs in FIG. 12B, which finally closes to the triangular form 22 given in FIG. 12F,. In all cases, the selectively grown GaN did not grow in a layer by layer mode, and thus the growth mode is not suitable for the growth of device structures.
A more recent result on GaN selective growth was given by T. Tanaka et al (TWN 1995 conference, Nagoya, Sep. 21, 1995). They tried to deposit GaN directly on sapphire substrates. Depending on the surface treatment condition (nitridization in ammonia or not) and growth temperature, they could obtain two different stripe forms. One is of the triangular shape and the other is a rectangular form. In both cases, no data hint to an improved crystal quality, and the quality is expected to be poor due to the missing GaN buffer layer. Furthermore the result shows that the shape strongly depends on substrate treatment, which can cause a problem of reproducibility.
Finally, our own study of selective growth of GaN showed similar problems as in Y. Kato et al. For the experiments, first 2 .mu.m of GaN (including a low temperature GaN layer) was deposited on sapphire, and then a 100 nm SiO.sub.2 film was sputtered on the GaN surface. The SiO.sub.2 film was partly removed by lithography and wet etching steps. The open areas formed stripes oriented in the (1120) direction. After this procedure, the samples were again loaded into the MOCVD apparatus machine. After heating the sample in an ammonia flow to 1130.degree. C. selective growth was initiated by introducing a flow of trimethyl gallium (TMG). In a cross sectional SEM picture, the obtained GaN stripe has a very rough surface morphology.
Therefore, a first object of the present invention is to provide a new method and layer structure which improves GaN crystal line quality drastically.
The method can be applied to many device structures. Therefore, a second object of the present invention is to provide a device formed on the improved gallium nitride, such a laser, UV-detector FET, HFET, HBT, HEMT and so on.